The demands of the construction industry for multifunctional, low cost construction materials has led to expanded use of composite substrates formed generally by compressing and heating a mat of particles and/or fibers combined with a resin binder and wax. While the most common fiber/particle components for such composites are cellulosic, such as wood particles, fibers, flakes or chips, there has also been significant research and development directed toward use of fibers/particles from other sources such as glass, synthetic polymers, carbon and inorganic fillers such as talc, alumina, silica, calcium carbonate and cementitious materials including fly ash and Portland cement. The most common composite substrates for use in construction today are those formed from particles, fibers, chips, flakes or other fragments of wood for the production of hardboard, medium density fiberboard, oriented strand board, particle board, plywood, and paper overlaid composites. Such composites are typically fabricated from a mixture of wood particles, fibers, flakes or chips with a binder, typically a thermosetting resin. The mixture is formed into a mat under wet-dry or dry process conditions and then compressed under heat and pressure into a dense composite substrate, typically in a sheet form. In some applications, such as in the manufacture of door skins, the mat is molded into a desired shape and/or provided with a smooth or textured surface during the thermal compression process. In related manufacturing processes paper is glued to the surface of the mat in the press. The manufacture of dense compressed composite substrates for use in the construction industry is well known in the art. See, for example, U.S. Pat. Nos. 3,164,511; 3,391,223; 3,940,230; and 4,241,133.
One important aspect of composite substrates destined for use in the construction industry is the quality and nature of the substrate surface. Many composite substrates are used in applications which require that the surface substrate be suitable for receiving finish coatings. Thus it is desirable that the substrate surface be hard, and substantially free from cracks, voids and porosity. Much effort has been directed to development of manufacturing techniques to obtain and assure high quality, ready-to-finish surfaces on composite substrates. Thus, for example, in the manufacture of finished door skins or exterior hardboard siding, a mat comprising wood pulp, resin binder and additives is compressed in a press between heated metal plates (platens) at a temperature of about 300.degree. F. to about 490.degree. F. at a pressure of about 500 to about 1500 psi for about 20 seconds to about 2 minutes. The resin binder is typically a thermosetting resin such urea/formaldehyde resins, phenol/formaldehyde resins, melamine/formaldehyde resins, acrylic resins, polyisocyanates or urethane resins. The mat is typically treated with a pre-press sealer to provide release from the hot press platen and thus optimize surface smoothness and minimize buildup on the press platens (metal plates). After the mat is pressed, typically to a predetermined stop thickness, the resulting board is further processed in a series of steps, including rehumidification, sizing, stacking, and transporting to a primer line for application of primer, and subsequent curing of the applied primer composition. With such current manufacturing techniques there is significant labor costs involved in transporting the composite board substrates from the press to the priming and curing stations. Further, there is significant capital and fuel costs associated with the required step of reheating and curing the primed composite boards.
Responsive to customer needs for reduced costs and improved quality of composite substrate construction materials, manufacturers of such materials have invested in significant research and development efforts to improve composite substrate manufacture. One goal of such efforts has been to develop a manufacturing process for composite substrates, particularly those formed from wood particulates and fibers, wherein the composite is formed with a primed/polymer coated surface in the press, thereby eliminating the subsequent steps of primer application and cure which are standard in current wood composite manufacturing operations. One such process is described in U.S. Pat. No. 5,635,748 wherein a polymer latex is applied as a foam on the surface of the mat, the foam is dried into a hardened layer which is thereafter crushed and set during pressing of the mat into a coated reconsolidated substrate. While that methodology is said to produce a primed composite board directly out of the press, the method requires an extra latex foaming step, and it requires an extra time/cost-consuming heating step similar to current manufacturing processes.
The present invention provides a cost efficient manufacturing process for manufacture of polymer coated (primed) composite substrates directly from the press without any extra latex processing or heating/drying steps. In accordance with one embodiment of this invention there is provided an improved process for manufacture of composite substrates having a high quality polymer coated surface directly out of the press. A fast-setting formaldehyde-free primer coating composition is applied to the surface of the compressible mat or to paper glued to the surface of the mat. The fast setting primer coating composition exhibits excellent "hold out" on the surface of the mat during subsequent mat compression between heated metal surfaces in a press. The primer coating composition is formulated to form a chemically crosslinked polymer matrix when or as it is applied to the surface. Pressing of the coated mat under standard conditions of elevated temperature and pressure produces a composite substrate having a smooth surface of low porosity, ideal for receipt of finish coating compositions. The present invention also enables the manufacture of coated paper wherein a fiber mat is coated and pressed into coated paper as part of the paper making process.
The primer coating composition comprises either a thermosetting polymer or a thermoplastic polymer and is otherwise formulated for rapid crosslinking/gel formation upon application to the surface of the mat. In one embodiment of the invention, the primer coating composition is formulated to undergo ionic crosslinking upon application to the compressible mat. In one preferred embodiment the primer coating composition comprises an anionically stabilized thermoplastic latex which undergoes a gel-forming pH dependent, ionic crosslinking reaction as it is applied to the surface of the mat. Alternatively, the primer composition can be a 2-component composition wherein the first and second components are capable of gel formation through ionic crosslinking when applied, for example, through a dual channel sprayer.
In another embodiment of the invention a top coat composition is applied over the applied primer coating composition before application of heat and pressure to the mat to form the polymer coated substrate. In one embodiment the top coat composition is a thermosetting latex composition which improves surface properties of the product polymer coated composition substrate and facilitates release of the composite from the heated metal surface, in the press. The top coat is preferably a formaldehyde free, low-temperature thermoset coating that functions both as a releasing agent and as an anti-metal-mark coating.
In yet another embodiment of the invention a release coat composition comprising a repaintable silicone polymer or a surfactant is applied over the primer coating composition to facilitate release of the polymer coated composite substrate from the press.
In still another embodiment of the invention the polymer coated composite substrate of this invention is prepared by a film transfer process. In that process, the primer coating composition is applied to a heated press platen, optionally over a first layer of a release agent and/or a thermosetting latex top coat composition, and the heated metal platen is thereafter contacted under pressure with the compressible mat optionally pretreated with an adhesive composition, to provide a compressed polymer coated composite substrate. The primer film transfer process can be employed with particular advantage in the manufacture of composite substrates in continuous belt-type presses.